Eyepiece optical system, optical apparatus and method for manufacturing eyepiece optical system

ABSTRACT

Provided is an eyepiece optical system including at least three lenses disposed in order from an observation object (Ob) along an optical axis. A final lens disposed closest to an eye point (EP) (corresponds to the third lens (L 3 ) in FIG.  1 ) is fixed when adjusting the diopter, and the following conditional expressions (1) and (2) are satisfied:
 
2.2&lt;| fe/fa |&lt;6.0  (1)
 
0.5&lt;| Re 2/ fa |&lt;5.0  (2)
         where fe denotes a focal length of the final lens, fa denotes a focal length of the total eyepiece optical system (EL), and Re2 denotes a radius of curvature of the eye point (EP) side lens surface of the final lens. When an optical surface is aspherical, a paraxial radius of curvature is used for calculation.

TECHNICAL FIELD

The present invention relates to an eyepiece optical system forobserving an image displayed on an image display element, that issuitable for an electronic view finder (EVF).

TECHNICAL BACKGROUND

An eyepiece optical system, which allows to observe an image displayedon a small image display element with high magnification, has beenproposed (e.g. see Patent Document 1).

PRIOR ART LIST Patent Document

Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-161915(A)

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In a conventional eyepiece optical system, a plate type protectiveoptical element is fixed on the side closest to the eye point in orderto protect the optical system and enhance the dust proof performance.However, when an object with high brightness is observed using a liquidcrystal display element for the image display element, ghosts and straylights are generated by the reflection on the display element andsurface of the optical element closest to the eye point.

With the foregoing in view, it is an object of the present invention toprovide an eyepiece optical system having good optical performance withminimum generation of ghosts and stray lights, an optical apparatusincluding this eyepiece optical system, and a method for manufacturingthe eyepiece optical system.

Means to Solve the Problems

To achieve this object, an eyepiece optical system according to thepresent invention includes at least three lenses disposed in order froman observation object along an optical axis, a final lens disposedclosest to an eye point is fixed when adjusting a diopter, and thefollowing conditional expressions are satisfied:2.2<|fe/fa|<6.00.5<|Re2/fa|<5.0where fe denotes a focal length of the final lens, fa denotes a focallength of the total eyepiece optical system, and Re2 denotes a radius ofcurvature of an eye point side lens surface of the final lens. When anoptical surface is aspherical, a paraxial radius of curvature is usedfor calculation.

In the eyepiece optical system according to the present invention, it ispreferable that the observation object is an image display element.

In the eyepiece optical system according to the present invention, it ispreferable that the image display element is a liquid crystal displayelement.

In the eyepiece optical system according to the present invention, it ispreferable that the following conditional expression is satisfied.0.60<Σd/fa<1.60where Σd denotes a distance on the optical axis, from an observationobject side lens surface of a first lens disposed closest to theobservation object to the eye point side lens surface of the final lens.

In the eyepiece optical system according to the present invention, it ispreferable that the following conditional expression is satisfied.0.60<Σd0/fa<1.20where Σd0 denotes a distance on the optical axis, from an observationobject side lens surface of a first lens disposed closest to theobservation object to the eye point side lens surface of the final lens.An air conversion length is used for an optical element that has norefractive power.

In the eyepiece optical system according to the present invention, it ispreferable that the following conditional expression is satisfied.5.00<fa<35.00(unit:mm)

In the eyepiece optical system according to the present invention, it ispreferable that all the lenses are made of plastic.

It is preferable that the eyepiece optical system according to thepresent invention includes, in order from the observation object alongan optical axis, a first lens and a second lens, wherein the diopter isadjusted by moving the first lens and the second lens, or only thesecond lens, along the optical axis.

An optical apparatus according to the present invention includes: anobjective lens; a picture element that captures an image formed by theobjective lens; an image display element that displays the imagecaptured by the picture element; and an eyepiece optical system forobserving the image displayed on the image display element, and theeyepiece optical system is any of the eyepiece optical systems describedabove.

A method for manufacturing an eyepiece optical system according to thepresent invention is a method for manufacturing an eyepiece opticalsystem including at least three lenses disposed in order from anobservation object along an optical axis, the method including: fixing afinal lens disposed closest to an eye point side, when adjusting adiopter; and disposing each lens in a lens barrel so as to satisfy thefollowing conditional expressions.2.2<|fe/fa|<6.00.5<|Re2/fa|<5.0where fe denotes a focal length of the final lens, fa denotes a focallength of the total eyepiece optical system, and Re2 denotes a radius ofcurvature of an eye point side lens surface of the final lens. When anoptical surface is aspherical, a paraxial radius of curvature is usedfor calculation.

Advantageous Effects of the Invention

According to the present invention, an eyepiece optical system havinggood optical performance with minimum generation of ghosts and straylights, an optical apparatus including this eyepiece optical system, anda method for manufacturing the eyepiece optical system can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram depicting a configuration of an eyepiece opticalsystem according to Example 1;

FIG. 2 shows graphs of various aberrations of the eyepiece opticalsystem according to Example 1 when the diopter is −1m⁻¹;

FIG. 3 is a diagram depicting a configuration of an eyepiece opticalsystem according to Example 2;

FIG. 4 shows graphs of various aberrations of the eyepiece opticalsystem according to Example 2 when the diopter is −1m⁻¹;

FIG. 5 is a diagram depicting a configuration of an eyepiece opticalsystem according to Example 3;

FIG. 6 shows graphs of various aberrations of the eyepiece opticalsystem according to Example 3 when the diopter is −1 m⁻¹;

FIG. 7 is a cross-sectional view of a digital camera; and

FIG. 8 is a flow chart depicting a method for manufacturing the eyepieceoptical system according to the present embodiment.

DESCRIPTION OF THE EMBODIMENTS

An embodiment of the present invention will be described with referenceto the drawings.

To protect an eyepiece optical system and enhance dust proofperformance, a fixed optical element has conventionally been disposed onthe side closest to the eye point, but if a plastic lens is used for theoptical element, a sufficient effect cannot be demonstrated even iftreated with anti-reflection coating, and reflection on the surface ofthis optical element stands out.

Therefore in the eyepiece optical system according to this embodiment,the final lens disposed closest to the eye point has a shape thatsatisfies the claims, whereby reflection on this lens surface issuppressed, and even if reflection is generated, an image of a glare dueto ghosts is formed outside the diopter that corresponds to the eye, sothat the reflection does not stand out, whereby the above problem issolved.

The eyepiece optical system according to this embodiment is suitable foran electronic view finder EVF (see FIG. 7). Therefore the observationobject is preferably an image display element. Here it is preferablethat the image display element is a liquid crystal display element. Aliquid crystal display element displays an image using the polarizingcharacteristic of liquid crystal, hence the range in which good displayluminous flux can be acquired is narrow. Generally this range isregarded to be ±10° from the direction perpendicular to the displaysurface, and light attenuation and change of tone are generated outsidethis range. This means that an eyepiece optical system, to observe aliquid crystal display element, requires some telecentricity. ThisCharacteristic is considered in the eyepiece optical system according tothis embodiment.

As shown in FIG. 1, the eyepiece optical system EL according to thisembodiment includes at least three lenses disposed in order from anobservation object Ob side, and the final lens disposed closest to theeye point EP is fixed when adjusting the diopter, and the followingconditional expressions (1) and (2) are satisfied.

In FIG. 1, the eyepiece optical system EL is constituted by threelenses: a first lens L1, a second lens L2 and a third lens L3 in orderfrom the observation object Ob. In this case, the final lens is thethird lens L3.2.2<|fe/fa|<6.0  (1)0.5<|Re2/fa|<5.0  (2)where fe denotes a focal length of the final lens, fa denotes a focallength of the total eyepiece optical system EL, and Re2 denotes a radiusof curvature of the eye point EP side lens surface of the final lens.When an optical surface is aspherical, a paraxial radius of curvature isused for calculation.

The conditional expression (1) specifies a ratio of the focal length ofthe final lens with respect to the focal length of the total eyepieceoptical system EL.

If the lower limit value of the conditional expression (1) is notreached, power of the final lens becomes strong and appearance drops dueto assembly errors. The curvature of field also worsens.

If the upper limit value of the conditional expression (1) is exceeded,ghosts and stray lights stand out when reflection is generated on thelens surface.

To demonstrate the effect of the above mentioned result with acertainty, it is preferable that the lower limit value of theconditional expression (1) is 3.0. To demonstrate the effect of theAbove mentioned result with certainty, it is preferable that the upperlimit of the conditional expression (1) is 5.0.

The conditional expression (2) specifies the ratio of the radius ofcurvature of the eye point EP side lens surface of the final lens, withrespect to the focal length of the total eye piece optical system EL.

If the lower limit value of the conditional expression (2) is notreached, strong external light from the outside is reflected on the lenssurface and ghosts and stray lights are generated, which may causediscomfort to the user. The curvature of field also worsens.

If the upper limit value of the conditional expression (2) is exceeded,ghosts and stray lights reflected on the image display element(observation element) Ob and the eye point EP side lens surface of thefinal lens stand out. In particular, if the eye point EP side lenssurface of the final lens has a positive radius of curvature, reflectionof the eye of the user stands out, which may cause discomfort. Thecurvature of field also worsens.

To demonstrate the above mentioned effect with certainty, it ispreferable that the lower limit value of the conditional expression (2)is 0.6. To demonstrate the above mentioned effect with certainty, it ispreferable that the upper limit value of the conditional expression (2)is 3.5.

In the eyepiece optical system EL according to this embodiment, it ispreferable that the following conditional expression (3A) is satisfied.0.60<Σd/fa<1.60  (3A)where Σd denotes a distance on the optical axis, from an observationobject Ob side lens surface of the first lens L1 disposed closest to theobservation object to the eye point EP side lens surface of the finallens.

The conditional expression (3A) specifies the ratio of the total lengthof the lens portion with respect to the focal length of the totaleyepiece optical system EL.

If the lower limit value of the conditional expression (3A) is notreached, a sufficient lens thickness cannot be secured, and satisfactorycoma aberration correction becomes difficult.

If the upper limit value of the conditional expression (3A) is exceeded,the total length of the optical system becomes long, and correction ofdistortion and curvature of field becomes difficult if miniaturizationis attempted.

To demonstrate the above mentioned effect with certainty, it ispreferable that the lower limit value of the conditional expression (3A)is 0.62. To demonstrate the above mentioned effect with furthercertainty, it is preferable that the lower limit value of theconditional expression (3A) is 0.63.

To demonstrate the above mentioned effect with certainty, it ispreferable that the upper limit value of the conditional expression (3A)is 1.40. To demonstrate the above mentioned effect with furthercertainty, it is preferable that the upper limit value of theconditional expression (3A) is 1.20. To demonstrate the above mentionedeffect to the maximum, it is preferable that the upper limit value ofthe conditional expression (3A) is 1.00.

In the eyepiece optical system according to this embodiment, it ispreferable that the following conditional expression (3B) is satisfied.0.60<Σd0/fa<1.20  (3B)where Σd0 denotes a distance on the optical axis, from the observationobject side lens surface of the first lens disposed closest to theobservation object to the eye point side lens surface of the final lens.Air conversion length is used for an optical element that has norefractive power.

The conditional expression (3B) specifies the ratio of the total airconversion length of the lens portion with respect to the focal lengthof the total eyepiece optical system EL.

If the lower limit value of the conditional expression (3B) is notreached, sufficient lens thickness cannot be secured, and satisfactorycoma aberration correction becomes difficult.

If the upper limit value of the conditional expression (3B) is exceeded,the total length of the optical system becomes long, and the correctionof distortion and curvature of field becomes difficult ifminiaturization is attempted.

To demonstrate the above mentioned effect with certainty, it ispreferable that the lower limit value of the conditional expression (3B)is 0.62. To demonstrate the above mentioned effect with furthercertainty, it is preferable that the lower limit value of theconditional expression (3B) is 0.63.

To demonstrate the above mentioned effect with certainty, it ispreferable that the upper limit value of the conditional expression (3B)is 1.10. To demonstrate the above mentioned effect with furthercertainty, it is preferable that the upper limit value of theconditional expression (3B) is 1.00. To demonstrate the above mentionedeffect to the maximum, it is preferable that the upper limit value ofthe conditional expression (3B) is 0.90.

In the eyepiece optical system EL according to this embodiment, it ispreferable that the following conditional expression (4) is satisfied.5.00<fa<35.00(unit:mm)  (4)

The conditional expression (4) specifies the focal length of the totaleyepiece optical system EL.

If the lower limit value of the conditional expression (4) is notreached, the lens diameter becomes large, and the correction ofdistortion and coma aberration becomes difficult accordingly.

If the upper limit value of the conditional expression (4) is exceeded,the total length of the optical system becomes long, and the worseningof distortion and curvature of field is inevitable if miniaturization isattempted.

To demonstrate the above mentioned effect with certainty, it ispreferable that the lower limit value of the conditional expression (4)is 10.00. To demonstrate the above mentioned effect with certainty, itis preferable that the lower limit value of the conditional expression(4) is 15.00. To demonstrate the above mentioned effect to the maximum,it is preferable that the lower limit value of the conditionalexpression (4) is 20.00.

To make the above mentioned effect with certainty, it is preferable thatthe upper limit value of the conditional expression (4) is 32.00. Tomake the above mentioned effect with further certainty, it is preferablethat the upper limit value of the conditional expression (4) is 29.00.To make the above mentioned effect to the maximum, it is preferable thatthe upper limit value of the conditional expression (4) is 27.00.

In the eyepiece optical system EL according to this embodiment, it ispreferable that at least one lens surface, out of the lens surfacesconstituting the first lens L1 to the final lens, is aspherical. Inparticular, if the observation object Ob side lens surface of the firstlens L1 is aspherical, the correction of coma aberration, astigmatismand distortion can be improved. If the eye point EP side lens surface ofthe second lens L2 is aspherical, the correction of distortion, comaaberration and spherical aberration can be Improved.

In the eyepiece optical system EL according to this embodiment, it ispreferable that all the lenses are made of plastic. Because of thisconfiguration, an aspherical surface can easily be formed, and asufficient aberration correction capability can be demonstrated forvarious aberrations, including coma aberration and distortion.

Since the eyepiece optical system EL includes, in order from theobservation object Ob, the first lens L1 and the second lens L2, and thefirst lens L1 and the second lens L2 are simultaneously moved or onlythe second lens L2 is moved along the optical system, the diopter can beadjusted without dropping the optical performance. If the final lens isfixed on the optical axis with respect to the observation object Ob whenadjusting the diopter, an optical element to protect the lens becomesunnecessary, ghosts and stray lights are prevented, and dust proofperformance can be enhanced.

FIG. 7 shows a digital camera CAM as an optical apparatus including theabove mentioned eyepiece optical system EL. The digital camera CAM isconstituted by an objective lens OL, a picture element C, such as CCDand CMOS, and an electronic view finder EVF. The electronic view finderEVF includes an image display element (observation object) Ob such as aliquid crystal display element, and the eyepiece optical system EL formagnifying and observing an image displayed on the image display elementOb.

In the digital camera CAM having the above configuration, light from anobject (not illustrated) is collected by the objective lens OL and formsan image of the object on the picture element C. The image of the objectformed on the picture element C is captured by the picture element C,and the image of the object captured by the picture element C isdisplayed on the image display element Ob. The user positions an eye onthe eye point EP, whereby the image of the object formed by theobjective lens OL can be observed in a magnified state via the eyepieceoptical system EL.

If the user presses a release button (not illustrated), the imagecaptured by the picture element C (that is, an image corresponding tothe image which is displayed on the image display element Ob and isobserved via the eyepiece optical system EL) is recorded in a memory(not illustrated) as an image of the object. In this way, the user canphotograph an object using the digital camera CAM.

According to the above mentioned digital camera CAM including theeyepiece optical system EL of this invention, a camera in which variousaberrations, particularly coma aberration and distortion, aresatisfactorily corrected, can be implemented.

Now an outline of a method for manufacturing the above mentionedeyepiece optical system will be described with reference to FIG. 8.First at least three lenses are disposed, in order from the observationobject along the optical axis, in a cylindrical lens barrel, so as tosatisfy the following conditional expressions (1) and (2) (step ST10).Then the final lens located closest to the eye point is disposed, so asto be fixed on the optical axis when adjusting the diopter (step ST20).2.2<|fe/fa|<6.00.5<|Re2/fa|<5.0where fe denotes a focal length of the final lens, fa denotes a focallength of the total eyepiece optical system, and Re2 denotes a radius ofcurvature of an eye point side lens surface of the final lens. When anoptical surface is aspherical, a paraxial radius of curvature is usedfor calculation.

As shown in FIG. 1, according to an example of the lens arrangement ofthe eyepiece optical system of this embodiment, a negative biconcavelens is disposed as the first lens L1. A positive biconvex lens isdisposed as the second lens L2. A negative meniscus lens having aconcave surface facing the eye point is disposed as the third lens L3.When adjusting the diopter, the first lens L1 and the second lens L2 aresimultaneously moved along the optical axis, while the third lens L3,which is the final lens located closest to the eye point, is fixed onthe optical axis with respect to the observation object Ob. Each lens isassembled in the lens barrel so as to satisfy the conditionalexpressions (1) and (2) (correspondence value of the conditionalexpression (1) is 4.02, and correspondence value of the conditionalexpression (2) is 0.70).

According to the method for manufacturing the eyepiece optical system ofthis embodiment, an eyepiece optical system having good opticalperformance with a minimum generation of ghosts and stray lights can beimplemented.

EXAMPLES

Each example according to this embodiment will now be described withreference to the drawings. Table 1 to Table 3 shown below are tables ofeach data of Example 1 to Example 3.

In each example, the d-line (wavelength: 587.5620 nm) and the g-line(wavelength: 435.8350 nm) are selected to calculate aberrationcharacteristics.

In [General Data] in each table, fe denotes the focal length of thefinal lens disposed closest to the eye point EP, fa is the focal lengthof the total eyepiece optical system, ω denotes an apparent angle offield (half angle of view) at −1 m⁻¹, and TL denotes a total length ofthe eyepiece optical system EL (distance on the optical axis from theobservation object Ob surface at −1 m⁻¹ to the lens surface closest tothe eye point EP in the eyepiece optical system).

In [Lens Data] in each table, the surface number is the sequentialnumber of the optical surface counted from the observation object Obside in the light traveling direction, r denotes the radius of curvatureof each optical surface, D denotes a distance on the optical axis fromeach optical surface to the next optical surface (or the eye point EP),νd denotes the Abbe number of the lens material at the d-line, nddenotes the refractive index of the lens material at the d-line,(variable) indicates the variable surface distance, “∞” in the radius ofcurvature r indicates a plane, and EP denotes the eye pointrespectively. The refractive index of air “1.0000” is omitted. When anoptical surface is aspherical, “*” is attached to the surface number,and the paraxial radius of curvature is shown in the column of theradius of curvature r.

In [Aspherical Data] in each table, the form of the aspherical surfaceshown in [Lens Data] is indicated by the following expression (a). HereX(y) denotes a distance along the optical axis direction from atangential plane at the vertex of the aspherical surface to eachposition on the aspherical surface at height y, r denotes a radius ofcurvature (paraxial radius of curvature) of the reference sphericalsurface, κ denotes a conical coefficient, and Ai denotes an asphericalcoefficient in the i-th order. “E-n” indicates “×10^(−n)”. For example,1.234E−05=1.234×10⁻⁵.X(y)=(y ² /r)/{1+(1−κ·y ² /r ²)^(1/2) }+A4×y ⁴ +A6×y ⁶ +A8×y ⁸  (a)

In [Conditional Expression] in each table, a value corresponding to eachconditional expression (1) to (4) is shown.

For the unit of the diopter, “m⁻¹” is used. Diopter X [m⁻¹] refers to animage formed by the eyepiece optical system EL located at a position 1/X[m(meter)] on the optical axis from the eye point EP. (The sign ispositive when the image is formed on the observer side from the eyepieceoptical system EL.)

In all the data values herein below, “mm” is normally used as the unitof the focal length f, radius of curvature r, surface distance D andother lengths, unless otherwise specified, but the unit is not limitedto “mm”, and another appropriate unit may be used since an equivalentoptical performance is obtained even if the optical system isproportionally expanded or proportionally reduced.

This description on a table is the same for all the examples, and istherefore omitted herein below.

Example 1

Example 1 will be described with reference to FIG. 1, FIG. 2 andTable 1. As shown in FIG. 1, an eyepiece optical system EL (EL1)according to Example 1 includes, in order from an observation object(image display element) Ob, a first lens L1 having negative refractivepower, a second lens L2 having positive refractive power, and a thirdlens L3 (final lens) having negative refractive power.

The first lens L1 is a negative biconcave lens. The observation objectOb side lens surface of the first lens L1 is aspherical.

The second lens L2 is a positive biconvex lens. The eye point EP sidelens surface of the second lens L2 is aspherical.

The third lens L3 is a negative meniscus lens having a concave surfacefacing the eye point EP.

The diopter is adjusted by simultaneously moving the first lens L1 andthe second lens L2 along the optical axis. At this time, the third lensL3 is fixed on the optical axis with respect to the observation objectOb.

Table 1 shows each data value of Example 1. The surface numbers 1 to 7in Table 1 correspond to optical surfaces m1 to m7 in FIG. 1respectively. In Example 1, the surfaces 2 and 5 are aspherical.

Each reference numeral and character in FIG. 1 is independent from otherexamples in order to simplify description. Therefore even if thereference numeral and character are the same as those in the drawing ofanother example, this does not mean that the configuration isnecessarily the same as the other example.

TABLE 1 [General Data] fe = −103.68(mm) fa = 25.79(mm) ω = 26.22° TL =34.5(mm) [Lens Data] Surface number r D νd nd 1 ∞ 16.2 *2 −51.98159 6.023.89 1.63550 3 32.92255 0.8 4 19.68874 7.0 56.21 1.52444 *5 −11.301583.0 6 28.66506 1.5 57.08 1.49108 7 18.02378 18.0 EP [Aspherical Data]Surface 2 κ = 4.89886, A4 = −0.20046E−03, A6 = −0.19561E−5, A8 =−0.23308E−08 Surface 5 κ = −0.01352, A4 = −0.33928E−04, A6 =−0.14109E−06, A8 = 0.40422E−08 [Conditional Expression] ConditionalExpression (1) | fe/fa | = 4.02 Conditional Expression (2) | Re2/fa | =0.70 Conditional Expression (3A) Σd/fa = 0.71 Conditional Expression(3B) Σd0/fa = 0.71 Conditional Expression (4) fa = 25.79(mm)

As the data in Table 1 shows, the eyepiece optical system of Example 1satisfies the conditional expressions (1) to (4).

FIG. 2 shows graphs of various aberrations (spherical aberration,astigmatism, coma aberration and distortion) of the eyepiece opticalsystem EL1 of Example 1 at diopter −1m⁻¹.

In each graph showing aberrations, Y1 indicates the height of theincident light when light emitted from the optical axis center of theobservation object Ob enters the tangential plane of the observationobject Ob side lens surface of the first lens L1 of the eyepiece opticalsystem EL1. d indicates an aberration curve at the d-line, and gindicates an aberration curve at the g-line. When neither d nor g isindicated, this means that the aberration curve is at the d-line. In thegraph showing astigmatism, the solid line indicates the sagittal imagesurface, and the broken line indicates the meridional image surface. Inthe graph showing coma aberration, “min” indicates “minute” of the angleunit. In the graph showing spherical aberration and the graph showingastigmatism, the unit of the abscissa is [m⁻¹] respectively, which isindicated as “D.” in the drawing.

The description on the graphs showing aberrations is the same for theother examples, where this description is omitted.

As each graph showing aberrations in FIG. 2 clarifies, in the eyepieceoptical system EL1 of Example 1, various aberrations including comaaberration and distortion are satisfactorily corrected, and excellentoptical performance is implemented.

Example 2

Example 2 will be described with reference to FIG. 3, FIG. 4 and Table2. As shown in FIG. 3, an eyepiece optical system EL (EL2) according toExample 2 includes, in order from an observation object (image displayelement) Ob, a first lens L1 having negative refractive power, a secondlens L2 having positive refractive power, and a third lens L3 (finallens) having positive refractive power.

The first lens L1 is a negative biconcave lens. The observation objectOb side lens surface of the first lens L1 is aspherical.

The second lens L2 is a positive biconvex lens. The eye point EP sidelens surface of the second lens L2 is aspherical.

The third lens L3 is a positive biconvex lens.

The diopter is adjusted by simultaneously moving the first lens L1 andthe second lens L2 along the optical axis. At this time, the third lensL3 is fixed on the optical axis with respect to the observation objectOb.

Table 2 shows each data value of Example 2. The surface numbers 1 to 7in Table 2 correspond to optical surfaces m1 to m7 in FIG. 3respectively. In Example 2, the surfaces 2 and 5 are aspherical.

TABLE 2 [General Data] fe = 103.82(mm) fa = 24.59(mm) ω = 27.30° TL =28.5(mm) [Lens Data] Surface number r D νd nd 1 ∞ 12.8 *2 −15.28582 3.823.89 1.63550 3 111.39414 0.8 4 25.04543 6.4 56.21 1.52444 *5 −12.006752.7 6 145.24769 2.0 57.08 1.49108 7 −78.20808 18.0 EP [Aspherical Data]Surface 2 κ = −1.60623, A4 = −0.38025E−03, A6 = −0.15854E−05, A8 =−0.95068E−08 Surface 5 κ = −0.13459, A4 = −0.70616E−04, A6 =0.80397E−07, A8 = 0.26710E−08 [Conditional Expression] ConditionalExpression (1) |fe/fa| = 4.22 Conditional Expression (2) |Re2/fa| = 3.18Conditional Expression (3A) Σd/fa = 0.64 Conditional Expression (3B)Σd0/fa = 0.64 Conditional Expression (4) fa = 24.59(mm)

As the data in Table 2 shows, the eyepiece optical system EL2 of Example2 satisfies all the conditional expressions (1) to (4).

FIG. 4 are graphs showing various aberrations (spherical aberration,astigmatism, coma aberration and distortion) of the eyepiece opticalsystem EL2 of Example 2 at diopter −1m⁻¹. As each graph showingaberrations in FIG. 4 clarifies, in the eyepiece optical system EL2 ofExample 2, various aberrations including coma aberration and distortionare satisfactorily corrected, and excellent optical performance isimplemented.

Example 3

Example 3 will be described with reference to FIG. 5, FIG. 6 and Table3. As shown in FIG. 5, an eyepiece optical system EL (EL3) according toExample 3 includes, in order from an observation object (image displayelement) Ob, a first lens L1 having positive refractive power, a secondlens L2 having negative refractive power, a third lens L3 havingpositive refractive power, and a fourth lens L4 (final lens) havingpositive refractive power.

The first lens L1 is a positive biconvex lens.

The second lens L2 is a negative meniscus lens having a concave surfacefacing the observation object Ob. The observation object Ob side lenssurface of the second lens L2 is aspherical.

The third lens L3 is a positive biconvex lens. The eye point EP sidelens surface of the third lens L3 is aspherical.

The fourth lens L4 is a negative meniscus lens having a concave surfacefacing the observation object Ob.

The diopter is adjusted by simultaneously moving the first lens L1 andthe second lens L2 along the optical axis. At this time, the third lensL3 and the fourth lens L4 are fixed on the optical axis with respect tothe observation object Ob.

Table 3 shows each data value of Example 3. The surface numbers 1 to 9in Table 3 correspond to optical surfaces m1 to m9 in FIG. 5respectively. In Example 3, the surfaces 4 and 7 are aspherical.

TABLE 3 [General Data] fe = 81.49(mm) fa = 24.10(mm) ω = 28.12° TL =29.8(mm) [Lens Data] Surface number r D νd nd 1 ∞ 8.6 2 229.82404 4.756.21 1.52444 3 −11.86646 5.1 *4 −6.11377 1.7 23.89 1.63550 5 −19.358432.0 6 46.64412 4.3 56.21 1.52444 *7 −13.42085 0.8 8 −24.41258 2.6 57.081.49108 9 −15.69482 18.0 EP [Aspherical Data] Surface 4 κ = 0.67273, A4= 0.12437E−03, A6 = 0.20583E−05, A8 = 0.81215E−07 Surface 7 κ = 1.35050,A4 = 0.13938E−03, A6 = 0.59071E−06, A8 = −0.10212E−08 [ConditionalExpression] Conditional Expression (1) |fe/fa| = 3.38 ConditionalExpression (2) |Re2/fa| = 0.65 Conditional Expression (3A) Σd/fa = 0.88Conditional Expression (3B) Σd0/fa = 0.88 Conditional Expression (4) fa= 24.10(mm)

As the data in Table 3 shows, the eyepiece optical system EL3 of Example3 satisfies all the conditional expressions (1) to (4).

FIG. 6 shows graphs of various aberrations (spherical aberration,astigmatism, coma aberration and distortion) of the eyepiece opticalsystem EL3 of Example 3 at diopter −1m⁻¹. As each graph showingaberrations in FIG. 6 clarifies, in the eyepiece optical system EL3 ofExample 3, various aberrations including coma aberration and distortionare satisfactorily corrected, and excellent optical performance isimplemented.

As described above, according to the present invention, an eyepieceoptical system having good optical performance with a minimum generationof ghosts and stray lights can be implemented.

The present invention has been described with configurationalrequirements of each embodiment to assist understanding of the presentinvention, but needless to say, the present invention is not limited tothe configurational requirements. The content of the description hereinbelow can be used within a scope that does not diminish the opticalperformance.

In each example, the eyepiece optical system is constituted by three orfour lens groups, but the present invention can also be applied to aconfiguration using a different number of lens groups, such as five orsix lens groups. In the configuration, a lens or a lens group may beadded to the side closest to the object, or a lens or a lens group maybe added to the side closest to the image. A “lens group” refers to aportion having at least one lens isolated by an air gap which changeswhen zooming is performed.

The lens surface may be formed to be a spherical surface or a plane, oran aspherical surface. If the lens surface is a spherical surface or aplane, lens processing, assembly and adjustment are easy, anddeterioration of the optical performance, due to an error in processing,assembly and adjustment, can be prevented. Even if the image planedeviates, the writing performance does not change very much. If the lenssurface is an aspherical surface, the aspherical surface can be anyaspherical surface out of an aspherical surface generated by grinding, aglass-molded aspherical surface generated by forming glass in anaspherical shape using a die, and a composite aspherical surfacegenerated by forming resin on the surface of the glass so as to be anaspherical shape. The lens surface may be a diffraction surface, and thelens may be a refractive index distributed lens (GRIN lens) or a plasticlens.

Each lens surface may be coated with an anti-reflection film, which hashigh transmittance in a wide wavelength region, in order to decreaseflares and ghosts, implementing a high optical performance at highcontrast.

EXPLANATION OF NUMERALS AND CHARACTERS

-   CAM digital camera (optical apparatus)-   OL objective lens-   C picture element-   Ob image display element (observation object)-   EL EL1 to EL3 eyepiece optical system-   L1 first lens-   L2 second lens-   L3 third lens-   L4 fourth lens-   EP eye point

The invention claimed is:
 1. An eyepiece optical system comprising atleast three lenses disposed in order from an observation object along anoptical axis, a final lens of the at least three lenses being a singlelens element disposed closest to an eye point and being fixed whenadjusting a diopter, and at least one lens in the rest of the at leastthree lenses being moved along the optical axis when adjusting adiopter, and the following conditional expressions being satisfied:2.2<|fe/fa|<6.00.5<|Re2/fa|<5.0 where fe denotes a focal length of the final lens, fadenotes a focal length of the total eyepiece optical system, and Re2denotes a radius of curvature of an eye point side lens surface of thefinal lens.
 2. The eyepiece optical system according to claim 1, whereinthe observation object is an image display element.
 3. The eyepieceoptical system according to claim 1, wherein the image display elementis a liquid crystal display element.
 4. The eyepiece optical systemaccording to claim 1, wherein the following conditional expression issatisfied:0.60<Σd/fa<1.60 where Σd denotes a distance on the optical axis, from anobservation object side lens surface of a first lens disposed closest tothe observation object to the eye point side lens surface of the finallens.
 5. The eyepiece optical system according to claim 1, wherein thefollowing conditional expression is satisfied:0.60<Σd0/fa<1.20 where Σd0 denotes a distance on the optical axis, froman observation object side lens surface of a first lens disposed closestto the observation object to the eye point side lens surface of thefinal lens; an air conversion length is used for an optical element thathas no refractive power.
 6. The eyepiece optical system according toclaim 1, wherein the following conditional expression is satisfied:5.00<fa<35.00(unit:mm).
 7. The eyepiece optical system according toclaim 1, wherein all the lenses constituting the eyepiece optical systemare made of plastic.
 8. The eyepiece optical system according to claim1, comprising, in order from the observation object along an opticalaxis, a first lens and a second lens, wherein the diopter is adjusted bymoving the first lens and the second lens, or only the second lens,along the optical axis.
 9. An optical apparatus, comprising: anobjective lens; a picture element that captures an image formed by theobjective lens; an image display element that displays the imagecaptured by the picture element; and an eyepiece optical system forobserving the image displayed on the image display element, the eyepieceoptical system being the eyepiece optical system according to claim 1.10. The eyepiece optical system according to claim 1, wherein each ofthe at least three lenses consists of a single lens element.
 11. Amethod for manufacturing an eyepiece optical system including at leastthree lenses disposed in order from an observation object along anoptical axis, the method comprising: constructing the eyepiece opticalsystem so that a final lens of the at least three lenses is a singlelens element that is disposed closest to an eye point side and is fixedwhen adjusting a diopter, and so that at least one lens in the rest ofthe at least three lenses is moved along the optical axis when adjustinga diopter; and disposing each lens in a lens barrel so as to satisfy thefollowing conditional expressions:2.2<|fe/fa|<6.00.5<|Re2/fa|<5.0 where fe denotes a focal length of the final lens, fadenotes a focal length of the total eyepiece optical system, and Re2denotes a radius of curvature of an eye point side lens surface of thefinal lens.